2 answers

There are potentially lots of external, 3rd party and proprietary apps that are used in conjunction with ODL, but to learn more about the apps/features that are shipped as part of the open source release you can start at the projects page on the wiki here

for each project, you can check out it's source code. Each project's main page should have a repository you can "git clone" from. Or you can find the project source code that's mirrored to github for example, here is the Group Based Policy's github landing page